This invention relates to a continuous process for industrially preparing chlorotrifluoroethylene by the reaction of 1,1,2-trichloro-1,2,2-trifluoroethane with zinc powder in an alcohol.
In the field of fluororesins and fluorine rubbers, chlorotrifluoroethylene (CTFE) is an important monomer for both polychlorotrifluoroethylene and various copolymers with, for example, ethylene, vinyl acetate or vinylidene fluoride.
As an industrial material CTFE is usually prepared from 1,1,2-trichloro-1,2,2-trifluoroethane (herein called "R-113"), and a prevailing method is to add R-113 to a dispersion of zinc powder in a lower alcohol, as shown in U.S. Pat. No. 2,831,901 by way of example.
Also it is possible to prepare CTFE by reacting R-113 with hydrogen in the presence of a copper, nickel or cobalt catalyst on a suitable carrier. This reaction seems to be favorable for industrial practice because of the simplicity of the operation, but actually this reaction is low in the selectivity to CTFE and the yield of same, and the product contains large amounts of impurities or by-products so that it is difficult to obtain high-purity CTFE suitable for polymerization by usual refining operations including distillation. Therefore, this reaction has not been employed as an industrial process.
The reaction of R-113 with zinc powder, which is represented by the following equation, is advantageous in that the reaction proceeds nearly stoichiometrically and that CTFE of sufficiently high purity can be obtained. ##STR1## In industrial practice this reaction is usually carried out by a batch process, and, as explained below, there are some problems which are almost inherent to a batch process.
(1) The reaction terminates as the entire quantity of initially charged zinc powder is consumed. Every time the reaction terminates, it is necessary to discharge zinc chloride formed by the reaction from the reactor together with the alcohol and newly charge the reactor with zinc powder and alcohol, followed by the replacement of the gas atmosphere in the reactor by an inactive gas such as nitrogen gas. These are troublesome operations and entail the loss of considerable time.
(2) The aforementioned inactive gas is liable to intrude into CTFE formed by the reaction.
(3) At the last stage of gradual consumption of zinc powder (after the conversion of about 80% of zinc in the reactor into zinc chloride) the rate of formation of CTFE becomes low so that a considerable change occurs in the flow rate of the reaction product, and accordingly there arises the need of varying some items of operation conditions.
There are proposals of carrying out the same reaction by a continuous process using a tray tower: e.g., U.S. Pat. No. 2,877,275, JP 47-45322 and JP 57-5207. However, when any of the hitherto proposed continuous processes is put into practice on an industrial scale there are some problems such as lowness of the conversion of zinc into zinc chloride and unstableness of operation by reason of difficulty of uniformly dispersing zinc powder in the continuously flowing alcohol and consequential accumulation of zinc powder in the tray tower or the piping, sometimes resulting in choking.